High speeds, extreme terrain and long vertical drops might be making the increasingly popular sport of mountain biking as risky as football, diving and cheerleading, according to a Canadian study.

The findings warn that taking two wheels to the trails invites the danger of a spinal injury. One of every six cases reviewed was severe enough to result in complete paralysis.

"People need to know that the activities they choose to engage in may carry with them unique and specific risks," says Dr Marcel Dvorak of the University of British Columbia in Canada.

"Helmets will not protect you from these injuries, nor will wearing Ninja Turtle-like body armour."

Previous studies had described both the range of injuries sustained by mountain bikers and the spinal injuries suffered across a variety of sports. But no one had evaluated the specific risks of spinal injury among mountain bikers.

Dvorak and his colleagues identified 102 men and five women who were seen at British Columbia's primary spine centre between 1995 and 2007 after mountain biking accidents.

Thrown over

The team couldn't calculate the risk of a spine injury among those who mountain biked, but they figured that over the 13-year study period, the annual rate was one in 500,000 British Columbia residents.

The riders accounted for 4% of all spine trauma admissions to the centre.

Surgery was required for about two-thirds of the mountain bikers, but the most devastating injuries were the 40% that involved the spinal cord. Of these, more than 40% led to complete paralysis.

"Wrist fractures and facial fractures are common" among mountain bikers, says Dvorak. "But spine injuries are the most severe with the most profound long-term consequences."

The majority of riders, he explained, were injured as a result of either being propelled over the handlebars or falling from great heights. In both scenarios, the result was often a severe impact to the head that triggered trauma down the neck and spine.

"The higher the jump or fall, the higher the risk," he said.

Perhaps surprisingly, the researchers found no relationship between helmet wearing and the overall severity of a rider's injuries.

"Helmets are good in preventing head injuries, but they do not in any way protect your neck," says Dvorak.